ABSTRACT
Ordovician and Lower Silurian sequences in the East Baltic contain the most complete record of evolution of Platystrophia following its first appearance in the late Arenig (Baltoniodus triangularis-B. navis conodont Biozone) until its extinction at the end of the Wenlock. Taxonomic diversity of the East Baltic Platystrophia was low during the late Arenig and Llanvirn, but increased substantially in the Llandeilo and Caradoc, when it became one of the most common brachiopods in the benthic assemblages. Platystrophia survived the terminal Ordovician extinction, but its significance in the Silurian brachiopod faunas is negligible in comparison with the Ordovician. Four studied species of Platystrophia from the Ordovician of the St Petersburg region (Russia) demonstrate remarkable variations in the morphology of the dorsal cardinalia, which can be used as a feature for species differentiation. Baltica represents the major centre for diversification and dispersion of Platystrophia in the Ordovician; however, related genera of plectorthids are absent in the older Billingenian Baltoscandian faunas and the first appearance of Platystrophia in the Baltic basin is possibly a result of immigration.
